don't allow save command to save both stdout and stderr to the same file (#9368)

# Description
It's not a good idea to save `stdout` and `stderr` to the same file from
`save` command directly.

Because it saves `stdout` and `stderr` in different thread, which leads
to in-consistent output. As replace, we can use `o+e` redirection to fix
the issue

# User-Facing Changes
```
❯ do -i { "aa" } | save foo.txt -e foo.txt
Error:   × input and stderr input to same file
   ╭─[entry #3:1:1]
 1 │ do -i { "aa" } | save foo.txt -e foo.txt
   ·                                  ───┬───
   ·                                     ╰── can't save both input and stderr input to the same file
   ╰────
  help: you should use `o+e> file` instead
```

# Tests + Formatting
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crates/nu-command/tests/commands/save.rs

@@ -84,13 +84,11 @@ fn save_append_will_not_overwrite_content() {
 }
 
 #[test]
-fn save_stderr_and_stdout_to_same_file() {
+fn save_stderr_and_stdout_to_afame_file() {
     Playground::setup("save_test_5", |dirs, sandbox| {
         sandbox.with_files(vec![]);
 
-        let expected_file = dirs.test().join("new-file.txt");
-
-        nu!(
+        let actual = nu!(
             cwd: dirs.root(),
             r#"
             let-env FOO = "bar";
@@ -98,11 +96,9 @@ fn save_stderr_and_stdout_to_same_file() {
             do -c {nu -c 'nu --testbin echo_env FOO; nu --testbin echo_env_stderr BAZ'} | save -r save_test_5/new-file.txt --stderr save_test_5/new-file.txt
             "#,
         );
-
-        let actual = file_contents(expected_file);
-        println!("{}, {}", actual, actual.contains("ZZZ"));
-        assert!(actual.contains("bar"));
-        assert!(actual.contains("ZZZ"));
+        assert!(actual
+            .err
+            .contains("can't save both input and stderr input to the same file"));
     })
 }